The ornamental design for a writinginstrument, as shown and described.
FIG. 1 is a perspective view showing our new design for a writing instrument;
FIG. 2 is a rear view of the writing instrument of FIG. 1;
FIG. 3 is a front view of the writing instrument of FIG. 1;
FIG. 4 is a right side view of the writing instrument of FIG. 1, the left side view being a mirror image thereof;
FIG. 5 is a top plan view of the writing instrument of FIG. 1;
FIG. 6 is a bottom plan view of the writing instrument of FIG. 1; and,
FIG. 7 is a perspective view of the writing instrument of FIG. 1 with cap removed.
The broken lines showing lines and portion of the writing instrument are included for the purpose of illustrating environment and form no part of the claimed design.